1. Field of the Invention
The present invention relates to the field of high speed, wide output current/voltage range laser diode/EAM driver circuits.
2. Prior Art
A laser diode (LD) is an opto-electronic device that provides an output light beam when the current through it goes over the lasing threshold current.
An electro-absorption-modulator (EAM) is an opto-electronic device that modulates the intensity of an incoming light beam based on the level of an electrical control voltage.
A bias current is a constant current that places an opto-electronic device just over the lasing threshold.
A modulation current is a switching current that brings the opto-electronic device further into the lasing mode.
A laser diode/EAM driver is a circuit that provides the control currents and voltages to an opto-electronic lasing device. It consists of a cascade of current switching stages and buffers that provide the appropriate amount of voltage and current gain, assuring optimal switching of the lasing device.
Edge Speed Improvement
The use of laser diodes with high extinction ratios requires laser drivers that are able to switch large currents. This results in the usage of large output transistors that have high feedback (Cbc—base-collector or Cgd—gate-drain) capacitances. These feedback capacitances (Cbc in FIG. 1 and FIG. 2) appear multiplied by the gain of the output switch (through the Miller effect—CMiller) at the input of the differential output switch, heavily loading the predriver and therefore significantly reducing the edge speed. The high current in the output switch results in a high voltage gain, resulting in a more pronounced Miller effect, and also a higher base-emitter charge storage capacitance (Cbe=Cpi) that adds to the capacitive loading of the predriver and further slows the output current/voltage waveform. There is a significant speed penalty for EAM drivers (that operate in a 50 Ohm environment) and laser drivers working in high impedance environments (Z0>25–30 Ohms).
Several methods have been used in the prior art to improve the edge speed. A first solution is to introduce emitter degeneration resistors (Rdegen) as presented in FIGS. 3a and 3b for LD and EAM drivers, respectively. The emitter degeneration decreases the voltage gain of the output switch and thus reduces the capacitive loading on the predriver due to the Miller effect. The emitter degeneration also reduces the loading on the predriver due to the lower effective base-emitter (Cpi) capacitance. Reducing both capacitive loadings on the predriver results in a significant speed-up of the driving voltage at the input of the output switch, and therefore a faster output waveform is obtained.
The foregoing method works well at high supply voltages or at low modulation currents where the voltage drop across the degeneration resistor does not significantly impact the headroom available to the switch. The major drawback of the emitter degeneration technique is that it requires a high voltage drop across the degeneration resistance. Specifically, to obtain a low voltage gain, the degeneration resistance needs to be a good fraction of the load resistance, making the circuit inoperable at high modulation currents and low supply voltages. In addition, the layout is not as compact, leading to more metal connections between transistors and degeneration resistors that add significant parasitic capacitances. These extra emitter capacitances enhance the peaking of the output waveform, requiring more RC compensation to be used. This slows down the edges, and thus part of the speed-up advantage given by the emitter degeneration is lost.
A second method used in the prior art to speed-up the driver is to add inductive peaking in the collector (drain) of the output switch as presented in FIG. 4. However, on-chip inductances capable of passing high current levels have high parasitic capacitances that short them at high frequencies. This is why in most cases the inductive peaking is done using bond-wire inductances (Lpeak). The major drawback of the inductive peaking is that it trades additional edge speed for more overshoot of the output waveform that gives supplementary deterministic jitter.
The inductive peaking works fairly well for the EAM drivers that use a symmetric differential output switch and have higher output impedances. In the case of laser drivers that in most cases require an open collector output switch and usually operate in much lower impedance environments (10–20 Ohm versus 50 Ohm for EAM drivers), adding series inductance to speed up the edges gives excessive overshoot that needs to be damped with additional RC compensation, which in turn slows down the edges. Also if the series inductance becomes high so that the L/R time constant becomes comparable with the data rate, the series inductance can even lead to a slowdown of the edge speed.
In the case of EAM drivers, the inductive peaking is performed by inductances not in the path to the EAM device, and therefore can be well controlled. In the case of laser drivers, the inductive peaking is difficult to control, as the inductive peaking element is in the path from the laser driver to the laser diode. This path is layout specific and varies from one assembly to another.
A third method used in the prior art to speed-up the driver is the neutralization of the Miller effect. This was done by adding two Miller effect cancellation capacitances (Ccancel) to the differential pair, each from the base of a respective device to the collector of the opposite device (in FIG. 5 from the base of Q1 to the collector of Q2 and from the base of Q2 to the collector of Q1). If these two capacitors closely match the base-collector capacitance (Cbc) of the output switch transistors (Q1 and Q2) they can provide a precise cancellation of the Miller effect. In real circuits, there will always be a mismatch that will reduce the Miller cancellation effect. Good matching can be achieved by using transistors of the same size as the output switch transistors for the Miller cancellation capacitances. The major drawback of this Miller cancellation technique is that the cancellation devices significantly increase the output capacitance of the driver and thus reduces the frequency of the ringing that appears during the switching process. If the ringing frequency comes close to the data-rate, it cannot be filtered-out by the SONET filter and will seriously increase the deterministic jitter. This Miller cancellation technique works well when the predominant slowing down effect is the capacitive loading of the predriver and not the output capacitance of the driver.
Temperature Compensation
The prior art has used various temperature compensation techniques to improve the laser diode switching behavior over temperature. One technique uses a laser driver that gives a modulation current that includes a positive temperature coefficient (PTAT) to compensate for the effects of the laser diode temperature increase.
Another technique uses a temperature dependent current (Itemp) to regulate the common-mode voltage at the bases of the switches Q1 and Q2 as shown in FIG. 6. By compensating for the temperature dependence of the VBE voltage of the output switch, the collector-emitter voltage (headroom) of the switch is maximized for a given power supply, assuring a higher edge speed.
Overshoot and Rise/Fall Time Control with Modulation Current Dependence of the Predriver Circuits
Actual laser drivers are required to operate over a wide modulation current range. Optimizing the rise/fall time and the overshoot/undershoot of the output waveform (current for a LD and voltage for an EAM) requires a modulation current dependence of the pre-driver currents.
Most of the prior art uses standard emitter followers in the pre-driver (Q3 and Q4 in FIG. 7). The drawback of this architecture is that it gives the same value of current for both turn-on and turn-off, leading to significant overshoot at turn-on when a high edge speed at turn-off is required.
An improvement of the standard emitter follower predriver architecture is presented in FIG. 8. It consists of using a dynamic emitter follower (devices Q3, Q4 and Q7, Q8) that has a different tail current at turn-on (Ief) and turn-off (Ief+Imod/M). This will bring a compromise between the rise/fall time and the overshoot. The drawback of this architecture is that it uses a constant turn-on current for all the modulation current levels, leading to excessive overshoot at low modulation currents. Another drawback is that the dynamic emitter followers require an additional driving voltage Vin* in phase opposition with the main input voltage Vin. The delay time from the Vin* input to the differential pair Q7, Q8 needs to be smaller than the delay time from the main input Vin to the Q3, Q4 emitter followers in order that the pull-up and pull-down current levels are set-up correctly. Achieving this delay time constraint requires a high current consumption in the additional driving path.
Another method to reduce the output rise/fall time is to use either symmetric or asymmetric dynamic coupled emitter followers (Q3, Q4 and Q7, Q8) that injects capacitive charging currents in the output switch, enhancing the peaking and therefore speeding-up the edges, as shown in FIG. 11. The advantage of this method over the traditional inductive peaking is that the edge speed is improved without worsening the ringing of the output current/voltage (without affecting the damping of the output RLC circuit). The drawback of this architecture is the increased supply current and large area of on-chip capacitance required for the dynamic coupling of the emitter follower. Furthermore, the overshoot cancellation is fixed and does not track with the external element layout.
On-Chip Versus Off-Chip Summation of Bias and Modulation Currents
The independent control of the bias and modulation current is achieved in most of the prior art by using two separate current sources. The summation of the bias current (Ibias) to the modulation current (Imod) is usually done off-chip by using a high value inductance (Lbias) to minimize the capacitive loading of the driver output by the bias circuitry (Cbias) as shown in FIG. 9.
The direct summation of the bias and modulation current at the driver output brings a severe edge speed penalty due to the capacitive loading of the driver output. One solution to this problem, as presented in FIG. 10, is to eliminate the separate bias circuit and to use a differential output pair (Q1, Q2) in which the devices switch between two on-state current levels. This eliminates any additional capacitive loading from the bias circuit. This architecture significantly improves the edge speed due to the switching between two on-state current levels, which is much faster than the on-off switching.
Predriver Current Control
Most prior art laser/EAM drivers use off chip control voltages to adjust both the voltage swing (Imod/N) and the pre-driver current levels (Imod/M) (see FIG. 7). These external adjustments are meant to optimize the switching performance when operating over wide modulation current and temperature ranges.